Resin Properties

In foam or balsa cored epoxy/fiberglass laminate boat the skins must take the load - the core has little strength or stiffness. A load on the hull (falling off a wave, for example) will generally put the loaded side laminate in compression and the opposite side in tension. Like rope, reinforcing fabrics are great in tension but poor in compression. It is the epoxy resin that must take the compression load. So, the first requirement of the epoxy here is that it be very stiff so that it does not buckle in compression. Most wood boat building epoxies have sufficient compressive strength for this when at room temperature. However, they rapidly loose stiffness and compressive strength as the temperature rises and, finally, about 145F they are totally inadequate and a sufficient compressive load will result in a catastrophic failure - first on the compression and then on the tension side. The laminate literally blows apart. So, the second requirement for this type of construction is that the epoxy maintain adequate compressive strength throughout the expected operating temperature range - generally considered to be up to about 175F (higher if the boat will be painted a dark color.)

As the temperature of a cured resin is increased the modulus (a measure of stiffness/resistance to bending) decreases. At first, the decrease is very gradual. As the temperature increases the rate of modulus decrease accelerates. Finally, at some point the rate of decrease hits a maximum and then begins to diminish with further increases in temperature. If one were to graph this with modulus on the vertical axis and temperature on the horizontal axis, both increasing as one moves away from the origin, a reverse "S" shape curve would result. At the point of maximum modulus rate change the curve would change from concave downward to concave upward. The lower temperature end of the curve is where the resin acts as a glass-like material while the resin acts as a rubber-like material. The point of maximum modulus change is called the Glass Transition Temperature (Tg pronounced as "tee-sub-gee"). (This is an important concept so take a break now, reread this paragraph and draw things out.)

One thing should now be obvious: One does not want to "operate" a laminated panel at a temperature above the Tg of the matrix resin. To be safe one wants to have the maximum operating temperature a few degrees below this. The operating temperature is the temperature of the skin laminate, which may be considerably higher than the temperature of the day if sunlight is beating down upon it. (See why light colors are preferred?) Something else ought to be obvious: One wants to use a high Tg resin for these laminates.

What may not be obvious is this: Not only must the resin have "a high Tg capacity" it must be cured at a temperature approaching its ultimate Tg or it will not fully cure. It may get hard enough at room temperature to hand-sand but it will not be fully cured. At some point you must raise the temperature of the laminate to no less than 30F of the ultimate Tg for several hours in order for it to fully cure. You can speed things up a bit by going a little over this but doing so will not raise the Tg further. So, the resin system must have the capacity to have an adequate Tg and it must be cured about this temperature. A laminate laid up at room temperature and later heated for ultimate curing is said to be "post-cured". Note that if a resin system does not have the capacity for a high Tg post curing will accomplish nothing. Wood/epoxy resins generally do not have the capacity to have high Tg's because the requirements stated in Part 1 are more important. In the 3rd and final part (next week) we'll explore the proper way to post cure a laminate.